It is often desirable to be able to determine the distance to a surface or object without actually touching the surface or object itself. In the field of process controls, for example, it may be desirable to determine the placement of machine parts, materials, sheets, or irregularly shaped objects without using tape measures, rules, or other normal contact like devices.
Apparatus has been devised in the prior art based upon the principle of projecting a beam of energy such as infrared energy or visible light towards the object in question and then receiving reflected energy from the object to determine the distance to the object at the point where the light impinged thereon. Devices such as are described in Swedish Pat. No. 424,271, Swedish pre-published patent application No. 80-03-606, Internation patent application WO81-02,628, European patent application No. 0,071,667, and German "Offenlegungsschrift" No. 32,22,462 all show apparatus of this sort. The prior devices described in the prior art above are of two general sorts. The first directs a beam of energy perpendicularly towards the surface in question and depends upon the scattering of light from the surface to detect reflections which are emitted therefrom at an angle. The difficulty with this type of sensor is that it does not work well or at all on highly reflective or specular surfaces since the energy from mirror like surfaces will be reflected directly back towards the source of light and will not be reflected at an angle towards the detectors. In the second type of system described in the references above a beam of energy is directed at an angle towards the surface and the detector arrangement is located on the opposite side of the vertical. Either the transmitting optics or the receiving optics includes a movable lens or mirror for rebalance so that the detector receives energy even when the surface is highly reflective. The difficulty with this type of system is that movable elements vastly increase the complexity and cost of the system and as the surface moves from one position to another between its extremes, the reflected light emitted from a specular surface will focus at a different position when the surface is diffuse. It is therefore quite difficult to devise a detector scheme which can receive the reflected light in a meaningful way regardless of the surface condition.